The present invention relates to pollution abatement and, more particularly, to a device for destroying gaseous pollutants by passing the pollutants through a plasma and possibly additional gas.
Gases that are hazardous or otherwise undesirable are produced by many commercial and industrial processes. Notable examples include oxides of nitrogen and sulfur, emitted, for example, from internal combustion engines and from power plants; chemical and biological agents such as sarin and tabun; and fluorine-containing greenhouse gases (perfluorocarbons) such as CF.sub.4, CHF.sub.3, C.sub.2 HF.sub.5, C.sub.2 H.sub.2 F.sub.4 and SF.sub.6 that are used in the fabrication of semiconductor devices. There are three general method to control emissions of these gases:
1. Control the processes that generate or use such gases to minimize their production or use. PA1 2. In the case of gases deliberately introduced to industrial processes such as semiconductor device fabrication, collect and recycle the emitted gases. PA1 3. Convert the gases to environmentally safer compounds. PA1 1. The smaller capacitance of a small cell makes it easier to drive at high frequencies. At higher frequencies, more discharge channels are created, so the plasma is more uniform. The smaller power supplies used with the smaller cells are simpler and more efficient than the large power supply that would be needed for a single large cell. PA1 2. A plurality of cells is easier to control than a single cell. It is easier and more efficient to control the concentrations of chemical species inside a plurality of small cells than inside a single large cell. According to the present invention, sensors are provided to measure the concentrations of gaseous species emerging from each cell and plasma conditions inside each cell. Power supply parameters such as frequency and voltage are adjusted adaptively, in accordance with the results of the measurements, to enhance the destruction of the unwanted species. PA1 3. A reactor made of a plurality of cells is modular. If one cell must be taken off line for maintenance, the reactor can continue to function.
The present invention addresses the third general method. Traditionally, the semiconductor industry has incinerated effluent gases. The burners used tend to be large, inefficient and expensive. Recently, it has been proposed to use plasmas, such as are used for generating ozone from oxygen, to destroy unwanted gaseous species. The high energy electrons of a plasma deliver their energy efficiently to atoms and molecules without heating the device which creates the plasma. The modification of the gas molecules is done by direct interaction with the electrons through electron attachment, dissociation or ionization, or through interaction with free radicals generated by the electrons.
There are two types of plasmas that may be used for pollution abatement: thermal plasmas and non-thermal plasmas. A thermal plasma is one that is in thermal equilibrium. Such plasmas may be generated by, for example, continuous RF or microwave energy. The particle energy in the plasma is a function of the plasma temperature, on the order of kT, where k is Boltzmann's constant and T is the plasma temperature. For typical thermal plasmas, the particle energy is on the order of electron volts. Non-thermal plasmas generate much higher electron energies, and therefore are characterized by more efficient energy transfer than thermal plasmas. The disadvantage of non-thermal plasmas is that they are more difficult to control and to keep uniform than are thermal plasmas.
Two types of non-thermal plasmas have been considered for pollution abatement: pulsed corona discharge and dielectric barrier discharge (DBD). In pulsed corona discharge, the plasma is generated between two electrodes by a pulse of high voltage across the electrodes, which creates a discharge in the gas between the electrodes. To prevent the creation of a single arc discharge which would carry the entire current and create a non-uniform plasma, the voltage pulse is kept short, on the order of tens of nanoseconds, and is repeated at a rate on the order of hundreds of times per second. The plasma discharge channels thus created do not have enough time to turn into an arc, so many discharge channels are created during the short lifetime of the pulse. Nevertheless, it is difficult to create a very uniform corona discharge. A representative U.S. patent describing a pulsed corona reactor is U.S. Pat. No. 5,490,973, to Grothaus et al.
In a DBD device, one or both of the electrodes are covered with an insulator so that the energy for the discharge is supplied capacitatively through the insulator. This limits the amount of energy that each discharge channel can receive. It therefore is possible to generate more channels and obtain a more uniform discharge.